GENEVA (Reuters) - The Saudi-led coalition has intensified air strikes on the Yemeni port of Hodeidah, possibly trapping civilians and hampering a humanitarian operation to import vital food and fuel supplies, the United Nations said on Friday.

Earlier this week, Yemeni government forces backed by Gulf Arab troops recaptured control of the Red Sea city of al-Mokha in a push that paved the way for an advance on Hodeidah, the country's main port city.

"Civilians were trapped and targeted during the al-Mokha fighting," U.N. High Commissioner for Human Rights Zeid Ra'ad al Hussein said in a statement. "There are real fears that the situation will repeat itself in the port of Hodeidah where air strikes are already intensifying."

He said possible war crimes had been documented with "alarming frequency" since the conflict between the Saudi-led coalition and Houthi rebels backed by Iran erupted almost two years ago.

A U.N. statement expressed concerns civilians in al-Mokha had been deliberately targeted by the rebel-linked gunmen during the battle for control of the port.

"Credible reports indicate that Houthi-affiliated snipers shot at families attempting to flee their homes in Houthi-controled areas," the statement said.

As the fighting shifted along the coast, the International Committee of the Red Cross (ICRC) said that civilians were already caught up in the fighting in Hodeidah, as well as in the towns of Taiz and Dhubab.

The aid agency pressed called for civilians to be allowed to leave for safer areas, and called for the wounded to be given access to medical care, in line with international law.

"We stand ready to deliver much needed aid to the civilian population," said Robert Mardini, ICRC regional director for the Near and Middle East.

Across Yemen, some 12 million people, roughly half the population, face the threat of famine and conditions are worsening, the United Nations warned on Wednesday as it appealed for $2.1 billion to fund food and other life-saving aid.

Jamie McGoldrick, U.N. humanitarian coordinator in Yemen, said the destruction of five cranes in Hodeidah port had forced dozens of vessels to line up offshore.

"We're trying to bring in four new mobile cranes to support the Hodeidah port, to try to ease the congestion there. That's something that we're in negotiation with Riyadh right now," McGoldrick said.